I didn't wake up for suhoor and slept till fajr prayer and didn't make niyat (intention) for fasting. But I made niyat after I woke up for fajr, does my fasting is valid, if not what expiation should be done?
In regards to your question that said:
I didn't wake up for suhoor and slept till fajr prayer …. does my fasting is valid?
The response of @Ansari is correct that he declared:
The very fact that you went to sleep intending to wake up for suhur is evidence of your niyyah.
Actually, your fasting would be correct if it was your intention since the last night of that in order to being fasting. Accordingly as you uttered “I didn't wake up for suhoor (Sahar) and slept till fajr prayer” (and by supposing this issue that it was your intention to do fasting tomorrow, then it would be valid.